The present invention relates to a syndiotactic vinylaromatic polymer having improved tensile strength. More particularly, the present invention relates to such a syndiotactic vinylaromatic polymer with induced orientation, and to a process for the preparation of such polymers.
Syndiotactic vinylaromatic polymers and a process for their preparation have been disclosed in U.S. Pat. No. 4,680,353, the teachings of which are incorporated herein by reference thereto. In the reference a polymerization catalyst is disclosed which is suitably employed in the preparation of vinylaromatic polymers having a syndiotactic configuration. By the term syndiotactic as used herein is meant that the relative intensity of racemic diads in the nuclear magnetic residence (NMR) spectrum is higher for such polymers than for polymers of identical monomers obtained by conventional free radical polymerization techniques. A preferred syndiotactic vinylaromatic polymer is syndiotactic polystyrene having a methyl ethyl ketone insoluable polymer content of at least about 75 weight percent.
Syndiotactic vinylaromatic polymers have significantly enhanced resistance to the effects of heat, that is the polymers possess elevated melting temperatures. For example, the crystalline melting point of syndiotactic polystyrene is approximately 270.degree. C. However, the polymers are extremely brittle making them ill-suited for use in certain applications such as molded objects where resistance to shock is desired. For example, normal syndiotactic polystyrene has a tensile strength of only about 1/2 that of general purpose amorphous atactic polystyrene. In order to provide improved physical properties it would be desirable to provide an oriented syndiotactic polystyrene.
It is previously known in the art to produce oriented fibers or films of thermoplastic molding polymers having improved tensile strength by drawing or otherwise pulling molten samples of the polymer into fibers (uniaxial orientation) or stretching films of the polymer in two directions (biaxial orientation) and cooling the same so as to lock in the desired molecular orientation. However, syndiotactic vinylaromatic polymers generally have glass transition temperatures which are on the order of hundreds of degrees less than the crystalline melt temperature thereof. Molten samples of the polymer are accordingly marked by a lack of viscosity in as much as they are at temperatures far in excess of the glass transition temperature. Such samples exhibit substantially reduced melt strength making it extremely difficult to introduce orientation by means of drawing, melt spinning or other previously known techniques.
The formation of crystalline structure in syndiotactic vinylaromatic polymers proceeds at a faster rate than in the corresponding isotactic polymers. Thus, upon cooling the molten polymer to a temperature less than the crystalline melting point under ambient conditions (referred to hereinafter as quiescent cooling) a highly crystalline but completely random polymer structure develops. For many polymers, the amount of crystallinity obtained in this manner, referred to herein as "normal crystallinity", may be measured by differential scanning calorimetry (DSC) or other suitable techniques. Generally, the normal crystallinity of a syndiotactic vinylaromatic polymer is from about 40 to about 60 percent, as determined by DSC. For syndiotactic vinylaromatic polymers the resulting crystals (normal crystals) are sufficiently large to affect light thereby rendering the polymer substantially optically opaque. Moreover, the presence of normal crystals results in a discrete X-ray diffraction pattern rather than a more diffused scattering that results for smaller crystal size materials. Such polymers are of course unsuited for certain applications requiring clarity such as films for packaging and other applications wherein the contents of a package are to be viewed.
It would be desirable if there were provided a process for producing an oriented, syndiotactic, vinylaromatic polymer.
It would further be desirable if there were provided a process for preparing an oriented, syndiotactic, vinylaromatic polymer wherein molecular orientation in one or two directions is provided.
It would further be desirable to provide an optically transparent, crystalline, syndiotactic, vinylaromatic polymer.